Coleman DGAT070BDD Flashes once

I have a 1996 Coleman DGAT070BDD which falls under the recall notice and has not be inspected yet. However, after a recent rain storm, the heat stopped working. I noticed a water leak had made it to the control board, and it had some corrosion on it. I replaced the control board but am receiving one flash stating that it had an ignition failure. I watch through the site glass to see that the ignitor and flame came on and went off 3 times before the trouble code flashed. I tried cleaning the flame sensor and checked continuity between the sensor tip and the solder joint on the control board which checked good. I even replaced the sensor. Is it possible the new board has a problem or is there something else I need to check.

1. Gas valve stays on if you supply power from the "W" screw terminal on the control board to the "+" terminal on the valve. However, control board still gives ignition failure code.
2. Transformer is receiving 120VAC and supplying 24VAC.
3. Jumped out limit switch directly above furnace to make sure it is not weak and opening prematurely. Gas valve is still shut off due to ignition failure.
4. New flame sensor has continuity between tip of sensor and solder joint on control board.

1. Removed and sanded ground connections. Then checked for continuity back to house ground.2. Checked flame sensor in series for 5 microamps. Reading never went above 0.3 microamps. However, while checking the amperage, the flame would ignite for only 8 sec. Prior to and after checking the sensor, the flame would ignite for around 30 sec before going out. If the sensor is disconnected, the flame will ignite for only 8 sec. The sensor seems to be working on some level. Odd though, acts like it's disconnected when the meter is hooked up in series. (Note: look at photo of meter to double check that I am using the correct settings - )3. Checked the centrifugal switch. 1st test - jumped out the switch and got two flashes to signal the switch was stuck closed. 2nd test - disconnected the switch and got three flashes to signal the switch failed to close (stuck open). 3rd test - initially disconnected the switch, and after a few moments of run time, jumped out the switch. The unit operated like it had prior to test. Switch appears to operate properly.4. Check upper and lower limit switches. Removed one wire from the limit switch. Each time, the board would signal four flashes for a limit switch that failed open.

Just in case it's helpful, here are the schematics that I have:Schematic for old control board located on sheet metal cover: Schematic for blend air switch: Schematic for new control board:

Second, you state you checked continuity of the FS wire back to the solder joint on the board. Clear this up for me, as I see the FS (yellow) wire going to a 9 pin molex along with the other gizmos. Further, is there any resistance on that wire?

Third, 6 seconds is the typical test duration for a flame sensor, although apparently Coleman is using 8 second duration. You were getting 30 second durations. I have to think about this for a bit...

And fourth, if I understand the symbology on your multimeter, it's looking for microamps AC and reading three. This is a DC current through the FS wire (flame rectified). If my rule of thumb is correct that would mean you're seeing 6 microamps DC, which is weak for a new sensor. Or an old sensor. Which must be engulfed in blue flame. Are you certain there is no positioning error (unlikely)?

Grasping at straws, only thing I can suggest right now is possibly a bad crimp or a break in the yellow wire. Is it greenish looking at the crimp?

I borrowed a Fluke meter from one of the service techs at work to retest the flame sensor. Got some interresting readings. The readings jumped around from 1.8 to 3.0 microamps and dropped to 0 at one point. I noticed that the flame is dancing around some and the readings seemed to corrospond to the flame dancing. Also, the flame stayed on more than 8 sec with this meter attached.

I did not adjust the position of the new sensor because it was in the same position as the old one. Could the burner unit be part of the problem? Here is a link to what it looks like:

Yes, flame characteristics could be a problem. The current rectifies through the flame, which conducts due to the voodoo of ionization. If you are seeing any yellow flame at all in the vicinity of the flame sensor rod, or floating flame, you may have a restriction in the orifice or the valve itself.

I'm presuming this is a typical Nordyne-style gas gun design. And I will buy that manual after all, this is just killing me.

So.

If you are confident in your skills, cut power to the unit, tape the switch down, and shut off the gas at the whip. Disconnect the whip et al from the gas valve and remove the gas valve. Remove the tube feeding the burner assembly (any intermediate steps being intuitively obvious). Inspect the valve opening for restrictions, i.e., spider webs.

Take pix and/or make diagrams before you start disassembly, you'd be surprised how easy it is to get turned around with these things.

Pulled the gas valve and inspected both ports. I could see the screens in both ports, and they were clean. The flame is blue. Every once in a while, where the flame is interrupted by the igniter or the sensor, there will be some yellow. Spun the sensor in the mount to move it to what looks like it would be more in the flame, but did not help. Checked to make sure the valve is getting 24VAC. I thought that the voltage was droping out on the valve when the blower would come on becuase the valve would shut. However, it shuts even if the blower is not engaged. Should I check the house voltage through a complete cycle (start to code flash) to see if the voltage is changing and causing the valve to shut...long shot?

Pulled the gas valve and inspected both ports. I could see the screens in both ports, and they were clean. The flame is blue. Every once in a while, where the flame is interrupted by the igniter or the sensor, there will be some yellow. Spun the sensor in the mount to move it to what looks like it would be more in the flame, but did not help. Checked to make sure the valve is getting 24VAC. I thought that the voltage was droping out on the valve when the blower would come on becuase the valve would shut. However, it shuts even if the blower is not engaged. Should I check the house voltage through a complete cycle (start to code flash) to see if the voltage is changing and causing the valve to shut...long shot?

I got the idea we had poor flame characteristics but this sounds nominal, blue right out to the flame spreader, nice and steady?

Then we're back to the low microamp values. I have no objection to taking that long shot but remain convinced our problem is in the flame sensor. You report transients from 0 to 3, and 3 is not enough to satisfy the controller. Or anyway not for long. Anything less than 5 is not acceptable, rule of thumb.

What are we not seeing on the burner assembly, or along that length of yellow wire, or at the molex, which is causing this? Do we have a resistance in that yellow wire? How does that wire connect to the board, exactly?